Abbreviations, names and symbols

Abbreviations
α-KG: α-ketoglutarate; α-KIC: α-ketoisocaproic acid; Aβ: amyloid-β = β amyloid; AMT: adsorptive mediated transcytosis; ANLS: astrocyte neuron lactate shuttle (hypothesis); apoE: apolipoprotein E; apoJ: clusterin; A − V difference: the difference between the concentrations in arterial blood entering and venous blood leaving the brain; BCAA: branched chain amino acid (large neutral amino acid); BCH: 2-aminobicyclo-(2,2,1)-heptane-2-carboxylic acid; BCKA: branched chain α-ketoacid (deaminated BCAA); BUI: brain uptake index; CSF: cerebrospinal fluid; ISF: interstitial fluid; LFER: linear free energy relations (Appendix C); LRP1, LRP2: low density receptor related proteins 1 and 2; MAO: monoamine oxidase; MeAIB, αMeAIB: α-(methylamino)isobutyric acid = N-methyl-α-aminoisobutyric acid; MW: molecular weight (dimensionless, MW of 12C is 12); NMR: nuclear magnetic resonance; PICALM: phosphatidylinositol-binding clathrin assembly protein; RAGE: receptor for advanced glycation end products; RMT: receptor mediated transcytosis; sLRP1: water soluble truncated form of LRP1; TfR: transferrin receptor; THO: tritiated water; VLDLR: very low-density lipoprotein receptor.

Symbols
β2H: solute hydrogen bond basicity (Appendix C); c (cisf, cplasma): concentration (in ISF or plasma); CL (CLBBB, CLperivascular): clearance (via blood–brain barrier or perivascular routes); CMRglc: cerebral metabolic rate of glucose; D: diffusion constant; F: Faraday constant; ΔGx/y: free energy change for the transfer from y to x (e.g. Eq. 5); J: flux; Jinf: water influx (measured using THO); Jnet: water net flux (in response to an osmotic gradient); Km: Michaelis–Menten constant for an enzyme reaction (e.g. by hexokinase) or transport process; Kt: apparent dissociation constant when Michaelis–Menten-like expressions (e.g. Eq. 28) are fitted to flux data for carrier transport; Kx/y: partition coefficient from y to x (e.g. Eq. 5); k: rate constant; keff: rate constant for efflux; keff,BBB: rate constant for efflux across the blood–brain barrier; N: amount (not to be confused with N, nitrogen); π2H : polarizability (Appendix C); P: permeability; PS: permeability surface area product; Pw,osmotic: water permeability calculated from net flux down an osmotic gradient; Pw,tracer: water permeability measured using tracers; R: universal gas constant; Rin: rate of input; Relim: rate of elimination; R1, R2, R3: different rates of input (Sect. 6); R2: excess molar refraction (Appendix C); S: surface area of microvessels; T: absolute temperature; T (Appendix E), period of time during which influx occurs; Tinf: influx (Appendix D); Teff: efflux (Appendix D); Tmax: maximum rate of transport in Michaelis–Menten type equations (Appendix D); Tnet: net flux (Appendix D); t: time; t1/2: half-life; Vbr: volume of distribution (Appendix E); V¯brain: conversion factor between the mass and volume of the brain, assumed to be 1 cm3 g−1 (Appendix E); VD: volume of distribution; Vx: molecular volume (Appendix C); z: charge on an ion, e.g. 1 for Na+.

Amino acid systems and transporters (see Fig. 19)23
system A: transport system for alanine and other amino acids, transports MeAIB; system ASC neutral amino acid transporter; system L: large neutral amino acid transport (4F2hc/Lat1 in the rat), transports BCH; system N: Na+-linked transport system for large N rich amino acids, e.g. glutamine, see SNAT; system xc−: cystine, glutamate exchange (mediated by 4F2hc/xCT); system XAG−: possible glutamate transport in luminal membrane; system y+: amino acid transport mediated by CAT1 at blood–brain barrier; system y+L: possible transport of neutral amino acids; ASCT2: Na+-dependent transporter for neutral amino acids (system ASC); ATA2: Na+-linked transporter—alanine preferring (system A); ATBo,+: neutral and basic amino acid transporter; CAT-1: cationic amino acid transporter 1; EAAT1, EAAT3: excitatory amino acid transporters 1 and 3 (abluminal membrane); Na+-LNAA: Na+ linked large neutral amino acid transporter (unknown gene); SNAT3: transporter for N rich amino acids (e.g. glutamine) linked to Na+/H+ exchange; 4F2hc/Lat1: heterodimeric transporter for large neutral amino acids, obligatory exchanger.

Other transporters and enzymes
BCRP: breast cancer resistance protein; gdh: glutamate dehydrogenase; GLUT1: glucose transporter 1 (not Na+-linked); g.s: glutamine synthetase; MCT1: monocarboxylate transporter 1; MRP1, MRP4: multidrug resistance proteins 1 and 4; OAT, Oat: organic anion transporter in HUMAN or other species; OATp, Oatp: organic anion transporting polypeptide in HUMAN or other species; OCT, Oct: organic cation transporter in HUMAN or other species; Pgp: p-glycoprotein; SLC (as in SLc22a8): gene name for a solute carrier (gene for Oat3); t.a: transaminase.